Light amplifier systems which are capable of projecting bright, dynamic images having color symbology superimposed on achromatic (blue-black and white) gray scale images are desirable for a variety of applications. The term "achromatic" is intended to include a gray scale which has a slight blue tint. The achromatic gray scale mode is applicable to large scale projection of television images. The color symbology mode is applicable to the display of symbology such as maps, titles, graphs, and grids. Such a color display can be used either separately or superimposed on the gray scale imagery for group presentations, head-up displays and the like. The color symbology can also be used to enhance the contrast of images such as produced by medical and scientific photography by converting the various levels of the gray scale into colors. The desirability of such systems has led to the development of liquid crystal light valves and in particular the photo-activated liquid crystal light valve. As is well known, this type of device is a thin film multilayer structure comprising a liquid crystal layer, a dielectric mirror, and a photoresponsive layer sandwiched between two transparent electrodes. A polarized projection beam is directed through the liquid crystal layer and to the dielectric mirror. A writing light input image is applied to the photoresponsive layer, thereby switching the electric field across the electrodes onto the liquid crystal layer to activate the liquid crystal. The activated liquid crystal in turn modulates the polarization state of the reflected projection beam. The reflected beam passes through an analyzer to create an intensity modulated beam which is projected by a lens onto a viewing screen. The intensity of the projected image has a point-to-point correspondence to the intensity of the writing light input image.
This operation is further described in U.S. Pat. No. 4,127,322, issued to A. D. Jacobson, et al, entitled "High Brightness Field High Brightness Full Color Image Light Valve Projection System" on Nov. 28, 1978, and U.S. Pat. No. 4,019,807 issued to D. D. Boswell, et al on Apr. 26, 1977, entitled "A Reflective Liquid Crystal Light Valve with Hybrid Field Effect Mode". Both patents are assigned to the assignee of the present invention.
The above-referenced Jacobson patent disclosed an optical system for projection of high brightness, full color television images onto a display screen from three field effect light valves. Provision was made to superimpose the color symbology image onto the full color image using a fourth light valve. By contrast, the present invention is directed to a system for display of black-white television images and/or a color symbology overlay which uses only a single light valve. The particular type of liquid crystal light valve used and disclosed in the Jacobson patent was also described in the article by Grinberg, Jacobsen et al entitled "Photoactivated Birefringent Liquid Crystal Light Valve for Color Symbology Display" and published in IEEE Transactions on Electron Devices, Vol. ED-22, No. 9, Sept., 1975, pp. 775-83. The device described in that patent and journal article projected a large scale display of color symbology and achromatic gray scale imagery from a single light valve operating in the field effect mode. That mode was achieved by homeotropic (perpendicular to the cell walls) alignment of the long axes of the liquid crystal molecules in the dark "off state" (electric field off the liquid crystal) and tilting of the long axes of the molecules in the transmitting "on state" (electric field switched on to the liquid crystal in response to the writing light applied to the photoresponsive layer). The homeotropic alignment was achieved by an alignment mechanism built into the cell walls containing the liquid crystal layer. During the course of further development of this device, it was discovered that the lifetime of the homeotropic alignment was limited to an extent that precluded its use in some commercial applications.
The above-referenced Boswell patent disclosed a hybrid field effect liquid crystal light valve for projection of monochromatic, i.e., blue, green or red, gray scale imagery at video rates. In operation, the dark off-state is created by rotation of the polarization of a twisted nematic structure of the liquid crystal and the transmitting on-state by the birefringence of the tilted molecules. Although possessing the advantages of high brightness, high contrast and time response suitably fast for television images, the device also has a number of limitations, among them being its inability to project black-white gray scale images as well as its inability to produce a color symbology mode on the monochromatic gray scale mode. Moreover, the device is difficult to fabricate because the thickness of the liquid crystal layer must be precisely tuned to match the color of the monochromatic projection light with which it is used in order to provide a high contrast image.
Bleha, Grinberg, Jacobson and Myer, in an article entitled, "The Use of the Hybrid Field Effect Mode Liquid Crystal Light Valve with Visible Spectrum Projection Light", published in the 1977 Society for Information Display (SID) International Symposium, Digest of Technical Papers, May, 1977, pp. 104-105, extended the design of the hybrid field effect device to include a liquid crystal layer having a thickness greater than was heretofore used. The article discloses that in a thickness range from 3 to 12 microns the thickness of the liquid crystal no longer has to be precisely tuned in order to display monochromatic gray scale images. The article also discloses that multicolor symbology can be produced by increasing the thickness of the liquid crystal layer to 6.0 microns or greater. However, as the authors point out in the article, the gray scale images are limited to monochromatic or non-neutral colors. Furthermore, because the time response of the liquid crystal layer varies as the square of its thickness, the multicolor symbology device is too slow to produce video rate images.